There almost certainly weren't 7 bombs - the Daily Mail was just parroting a Wall Street Journal report that turned out to be probably untrue.
"Counterterrorism officials found five additional suspect devices around the Boston area on Monday, but a law-enforcement official later said closer examinations led them to doubt that they were bombs. An earlier version of this article cited people briefed on the investigation saying officials believed they were undetonated explosive devices."
They said on Tuesday that there were only two bombs. No other unexploded devices were found.